1. Field of the Invention
This invention relates to an electromechanical drive system that includes a transmission, a brushless variable speed electric motor having velocity sensing instrumentation, and an industrial programmable controller. More particularly, this invention pertains to such a system for operating the displaceable carriages of way and transfer units used in automatic machining operations.
2. Description of the Prior Art
Conventional electromechanical slide drive systems employ two motors. One operates at high speed for rapid traverse of the distance between the position of the slidable carriage when the workstation receives the workpiece from the conveyor system to its position before the cutting operation begins. The other motor operates at a much lower speed to feed the cutting tool slowly toward the workpiece during the machining operation. Typically, the two motors operate through an elaborate mechanical transmission employing clutches and brakes to selectively connect the motors through appropriate gear reduction ratios to a lead screw that produces the linear motion of the slide. During the operating cycle, the transitions from rapid forward speed to feed speed, from feed speed to rapid reverse, and from reverse to dwell are controlled by a number of electromechanical limit switches. The clutches, brakes and limits switches and the many gears, belts, bearings used in such systems are susceptible of premature failure and high rates of wear.
Furthermore, the uncertain nature of brake performance requires that the slide position defining conclusion of the high speed traverse and commencement of the low speed feed be located substantially ahead of the point where the cutting tool engages the workpiece. The location of the transition from high speed to feed speed has been found to vary in drive systems according to current practice as much as three-eights of an inch from the intended point of transition. As a consequence of this wide tolerance range and the need to slow the slide unit well away from the workpiece, a considerable amount of lost time is spent in transporting the cutting tool at the slow feed rate into engagement with the workpiece.
Conventional d.c. motors can produce variable speed, high torque and precise control, but the brushes of such motors require frequent maintenance, particularly in the environment of a manufacturing plant. For this reason it is preferable that a brushless motor be used to power the drive system. In addition to providing reliable maintenance-free operation, the preferred motor must produce sufficient torque to move heavy objects at high speed and to feed them against the force of the cutting tool. Furthermore, the motor must develop a high rate of acceleration in order that the cutting tool can be moved quickly toward the workpiece and returned to its rest position. Of course, it is essential that the motor conforming to these specifications consumes as little power as possible.
With conventional slide drives the way units must be decelerated from fast speed to feed speed approximately one-quarter to three-eights of an inch from the work surface to prevent the cutting tool from impacting the workpiece due to slower than normal braking action. The result of premature slowdown is known as "air cutting", i.e., the process during which the tool is advanced slowly enough to cut metal but in reality is merely rotating in air. It is preferable that a slide drive system initiate the feed speed position of the motion cycle closer to the cutting tool, perhaps within 0.002 inches of the work surface.
In conventional slide drives, the feed speed is limited to one of a number of discrete speeds determined by the available gear ratios of the transmission. In order to vary the rate of feed speed, the gear ratios of the transmission must be changed. If the feed rate were infinitely adjustable and could be varied by a simple programming modification made to the control system, variations in the material of the workpiece and quality of the cutting tool could be accommodated to facilitate production. For example, if the hardness varied between batches of workpieces or the sharpness of the cutting tool deteriorated as machining time accrued, conventional slide drives could not automatically change the feed rate. By simple modifications to the programmed feed rate, the control according to this invention can deal with such subtle yet predictable process variations.
The controls for systems heretofore available require complicated mechanical assemblies whose reliability is low and whose repair is difficult, time consuming and costly. Further, these controls are inflexible and not readily adaptable to permit changes in a production line that require alterations in the forming process, variations in the workpiece material or the processing thereof. Highly automated, modern manufacturing production lines require great flexibility in adapting the controls of the forming tools to a wide variety of conditions, processes, and dimensional changes. It is preferable that the controls for operating the mechanical drive be suitable with this requirement for simple and easy changeover. Furthermore, the controls should be adaptive controls to adjust the working cycle to variations in dimensions or machinability between workpieces of the same kind.